Missed this post, sorry. Showed them to my SO and it could be a number of things, but it mostly looks like an open wound with perhaps a mild infection. You obviously need a vet to check on her, but in the meantime it probably won't hurt to rub some disinfectant on it with some gauze: you can use either Betadine dilluted 1:1 with water, or undiluted chlorhexidine. Some healing ointment would probably also help: we use Blastoestimulina in Spain but you should probably ask your vet for an equivalent that is also cat-safe.Slightly depressing side note: Does anyone know what this could be? (Slight graphic warning, nasty looking ulcer thing. Not a closeup though!)
Indoor-only cat, and it does look like the sore has gotten bigger (though it doesn't seem to cause her pain or trouble eating). We're in touch with a vet but because neither of our cats like to be picked up AND she's quite old and skinny as it is (she's been skinny for a few years though, no idea why) we're worried about even transporting her, let alone any treatment they might want to give her for it.
We're trying to organise an in-home visit and they're also suggesting giving her something to calm her down so we can bring her in, but my wife (they're her cats initially) is paranoid any kind of medication could kill the poor girl :(
I'm not one to normally go for internet advice or whatever, but if there's a way we can treat the sore/her without causing her anxiety/stress, it would be the best option I think.
From where it's located I'm assuming the answer is "no", but you might want to check if it's one of her lower teeth that's causing the wound in the upper lip (sometimes their teeth, particularly fangs, can cause sores like that in the opposite lip). I also sent the pics for her to forward to our vet, in case he has some more insight.
All in all the sore itself is probably nothing to worry about, but a vet should still see it to rule out that it's caused by fungi or some other underlying pathology.
Edit: Our vet messeged my SO with (translated) "eosinophilia: treat with cortisone and treat for fleas". Sorry for the sparseness, we can't ask for much more for free and at 23:30. :D
Edit 2: Our vet sent an even more cryptic message: "DAPP". My SO snapped at him to stop being the freaking Riddler and he elaborated "dermatitis alérgica a la picadura de pulga" (flea bite allergic dermatitis). Could a flea have bitten her? Our vet has a pretty amazing track record of being right...